1. Field of the Invention
The present invention relates to a surface acoustic wave device having an unbalanced-to-balanced (“balun”) conversion function, for use in, for example, filters and other apparatuses, and a communication device including such a surface acoustic wave device.
2. Description of the Related Art
In recent years, in order to obtain high performance high-frequency circuits to be used in communication devices such as portable telephones, there is a movement to make such high-frequency circuits balanced ones. In this case, the electronic components of the high-frequency circuits which can accommodate balanced circuits are needed. Although there is a balun as an electronic component having terminals for conversion between unbalanced and balanced circuits, since the number of electronic components increases, the development of electronic components having an additional unbalanced-to-balanced conversion function has been carried out. In the electronic components having an unbalanced-to-balanced conversion function, it is required that the amplitude balance is 0 dB and the phase difference showing the phase balance is close to 180 degrees.
A surface acoustic wave device, which can be made smaller as the frequency to be handled in the high-frequency circuit increases, can be used as the above-mentioned electronic component. Although not illustrated, the above surface acoustic wave device contains one or a plurality of comb-shaped electrode portions, that is, interdigital transducers hereinafter referred to as IDTs) and two reflectors on the left and right sides along the propagation direction of a surface acoustic wave between which the IDTs are sandwiched, for example, along the propagation direction of a surface acoustic wave.
The IDTs are made of a metal thin film of aluminum, and the IDTs function as a surface acoustic wave converter such that an input electrical AC signal is converted into a surface acoustic wave (elastic energy) to cause the surface acoustic wave to be propagated on the piezoelectric substrate, such that the propagated surface acoustic wave is converted into an electrical signal to output the signal. The reflectors reflect a propagated surface acoustic wave in the direction from which the surface acoustic wave came.
In such IDTs, the signal conversion characteristics and passband can be determined by setting the length and width of each electrode finger, the spacing between adjacent electrode fingers, and the cross width, that is, the facing length of adjacent electrode fingers in the comb-shaped electrodes. Furthermore, in the reflectors, the reflection characteristics can be set by adjusting the width of each electrode finger and the spacing between electrode fingers.
Also in such surface acoustic wave devices, the devices having an unbalanced-to-balanced conversion function are under development and being practically used. When the filtering characteristics having a high attenuation value is required outside the passband, a multi-stage construction in which surface acoustic wave elements which are longitudinally coupled are connected in series, as described in Japanese Unexamined Patent Application Publication No. 10-117123, is used.